Position detector

Abstract

 A position detector is provided with an encoded element (26) having indicia (30, 32, 34) representative of a bit pattern of a pair of bits of a Gray code and a pair of sensors (38, 40) responsive to this indicia. The encoded element is capable of being mechanically coupled to a movable member and the sensor are spaced apart by a distance corresponding to a shift in the bit pattern between two bits of the Gray code such that movement of the encoded element past the sensors is used to produce two bits of the Gray code. An additional encoded element (44) and sensors (50, 52) are used to produce additional bits of the Gray code.

Description

[0001] This invention relates to position detectors and, more particularly, to such detectors which produce a digital indication of the position of a movable mechanical member.

[0002] Many mechanical devices require the reliable measurement of the position of a movable element. For rotating elements such as the shafts of dynamoelectric machines, the angular position of the shafts can be deter­mined through the use of commercially available shaft encoders. However, in certain applications such encoders are not acceptable. For example, position sensors required for the rotating shafts of brushless DC motors or AC starters for aircraft engines, must meet severe size and environmental requirements.

[0003] Magnetic types of position detectors are mechani­cally rugged. These types include permanent magnet voltage pickups or variable reluctance types of sensors. Voltage pickup types will not operate at very low speeds. There­fore, motor applications requiring operation at or near zero revolutions per minute must use the variable reluc­tance type of sensor.

[0004] Reluctance sensors typically use a toothed wheel which passes near an AC excited coil, varying the reluc­tance and therefore the inductance of the coil. The inductance change produces a detectable change in current or frequency of an oscillator which is resolved into a discrete logic level. A typical four bit sensor uses four toothed wheels and four coils to produce a digital word. The four bit word would provide a detector resolution of one part out of 16.

[0005] It is well known that linear and rotary encoders use so called Gray codes to avoid ambiguous readings as the bits change between digital words. Gray codes (which may have redundant bits) change only one bit at a time between adjacent positions. That is, adjacent words are identical except for one bit position.

[0006] This invention provides a position detector which produces a digital output in the form of Gray codes, but does not require a separate encoded element and sensor for each bit of the Gray code data words.

[0007] Position detectors constructed in accordance with this invention include an encoded element which has indicia representative of a bit sequence of a pair of bits in a Gray code and is capable of being mechanically coupled to a movable member. A pair of sensors are positioned adjacent to the encoded element such that the sensors produce digital signals in response to the indicia. The sensors are mechanically spaced from each other by an amount which corresponds to the amount of shift of the bit sequence between the pair of bits in the Gray code.

[0008] For four bit resolution, a second encoded ele­ment, which is also capable of being mechanically coupled to the movable member, is included. Two additional sensors are positioned adjacent to the second encoded element and mechanically spaced apart from each other by an amount corresponding to a shift of a second bit sequence between a second pair of bits. This second pair of sensors is responsive to indicia on the second encoded element which is representative of the second bit sequence.

[0009] In an alternative embodiment, only one sensor is associated with the second encoded element and a third encoded element is provided to operate in conjunction with the last sensor.

[0010] By using Gray codes which include identical but shifted bit sequences in two or more bits, this invention reduces the number of required encoded elements for a given position resolution. This is accomplished through the use of mechanically spaced sensors which respond to indicia representative of the bit sequence on a single encoded element that is coupled to the movable member.

[0011] The invention will become more readily apparent from the following description of the preferred embodiments thereof, shown by way of example only, in the accompanying drawings wherein:

Figures 1 and 2 are logic diagrams illustrating the bit sequences of four bit Gray code data words produced by position detectors of the present invention;

Figures 3, 4 and 5 are schematic diagrams of toothed wheels and associated sensors which may be used to produce the bit sequences represented by Figures 1 and 2;

Figures 6, 7 and 8 are schematic diagrams of linear encoded elements and associated sensors, which may be used to produce the Gray code bit sequences represented by Figures 1 and 2; and

Figures 9, 10 and 11 are schematic representa­tions of encoded elements and associated sensors, used to produce the Gray code illustrated in Table IV below.

[0012] Position detectors constructed in accordance with this invention produce digital data word outputs corre­sponding to the mechanical position of an associated movable member. The data words are in the form of Gray codes having at least two bits which contain identical bit sequences that are shifted by a predetermined number of position indications. Four bit Gray codes having two pairs of such identical repetitive bit sequences are illustrated by the logic diagrams of bit sequences indicated in Figures 1 and 2. A four bit code provides one in 16 resolution as illustrated by the position indications ranging from zero to 15. These position indications may correspond to angular positions of a rotating shaft, or specific positions of a linearly movable member.

[0013] In Figure 1, bit sequences 10 and 12 are seen to be identical to each other but shifted by four position indication numbers. Similarly, bit sequences 14 and 16 are identical to each other but shifted by four position indication numbers. Starting at position indication number 9 bit sequence 10 is shown to include two zero bits, three one bits, three zero bits, two one bits, three zero bits, and three one bits. Bit sequence 12 is seen to be identi­cal if one starts at position indication number 5. Bit sequences 14 and 16 both include eight zero bits followed by eight one bits and bit sequence 16 is shifted by four position indication numbers with respect to bit sequence 14.

[0014] Referring to Figure 2, starting at position indication number 1, bit sequence 18 is shown to include two one bits, two zero bits, two one bits, three zero bits four one bits, and three zero bits. This same bit sequence is found in bit sequence 20 starting at position indication 9. Bit sequences 22 and 24 are similar to bit sequences 14 and 16 in Figure 1.

[0015] Tables I and II illustrate all four bit Gray codes having sequences corresponding to Figures 1 and 2 respectively.

TABLE I

Four Bit Gray Codes Having Bit Sequences Corresponding to Figure 1
N	GC1	GC2	GC3	GC4	GC5	GC6	GC7	GC8
	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA
0	0000	0000	0000	0000	0000	0000	0000	0000
1	0001	0001	0001	0001	0001	0001	0001	0001
2	0011	0011	0011	0011	0011	0011	0011	0011
3	0010	0010	1011	0111	0111	0111	1011	1011
4	0110	1010	1001	0110	0110	0101	1010	1010
5	0100	1000	1101	0100	0010	1101	1000	0010
6	0101	1001	0101	0101	1010	1001	1001	0110
7	0111	1011	0111	1101	1011	1011	1101	0111
8	1111	1111	1111	1111	1111	1111	1111	1111
9	1110	1110	1110	1110	1110	1110	1110	1110
10	1100	1100	1100	1100	1100	1100	1100	1100
11	1101	1101	0100	1000	1000	1000	0100	0100
12	1001	0101	0110	1001	1001	1010	0101	0101
13	1011	0111	0010	1011	1101	0010	0111	1101
14	1010	0110	1010	1010	0101	0110	0110	1001
15	1000	0100	1000	0010	0100	0100	0010	1000

TABLE II

Four Bit Gray Codes Having Bit Sequences Corresponding to Figure 2
N	GC9	GC10	GC11	GC12	GC13	GC14	GC15	GC16
	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA	DCBA
0	0000	0000	0000	0000	0000	0000	0000	0000
1	0001	0001	0001	0001	0001	0001	0001	0001
2	0011	0011	0011	0011	0011	0011	0011	0011
3	0010	0010	0010	0010	1011	0111	0111	1011
4	0110	0110	1010	1010	1001	0110	0101	1010
5	0111	0111	1011	1011	1101	0100	1101	1000
6	0101	1111	1001	1111	1111	0101	1111	1001
7	0100	1011	1000	0111	0111	1101	1011	1101
8	1100	1010	1100	0110	0101	1100	1001	1100
9	1110	1110	1110	1110	0100	1110	1000	1110
10	1111	1100	1111	1100	1100	1111	1100	1111
11	1101	1000	1101	0100	1110	1011	1110	0111
12	1001	1001	0101	0101	0110	1001	1010	0101
13	1011	1101	0111	1101	0010	1000	0010	0100
14	1010	0101	0110	1001	1010	1010	0110	0110
15	1000	0100	0100	1000	1000	0010	0100	0010

[0016] Notice that each of the Gray codes illustrated in Tables I and II includes two basic bit sequences and each of these bit sequences is found in two bits of the data words. For the purposes of this discussion, corresponding bits are defined as bits having identical bit sequences which are shifted by a given number of position indications. Table III lists the corresponding bit sequences in the Gray codes GC1-GC16 illustrated in Tables I and II.

TABLE III

Corresponding Bit Sequences In Gray Codes GC1-GC16
Code	Corresponding Sequence Bits
GC1	D-C	B-A
GC2	D-C	B-A
GC3	D-B	C-A
GC4	D-C	B-A
GC5	D-B	C-A
GC6	D-A	C-B
GC7	D-C	B-A
GC8	D-A	C-B
GC9	D-C	B-A
GC10	D-B	C-A
GC11	D-C	B-A
GC12	D-A	C-B
GC13	D-B	C-A
GC14	D-C	B-A
GC15	D-A	C-B
GC16	D-C	B-A

[0017] It should be noted that in Table II above, Gray codes GC10, GC12, GC14 and GC16 include bit sequences which have logic levels which are the inverse of the bit sequenc­es 18 and 20 illustrated in Figure 2.

[0018] This invention takes advantage of the symmetry of the Gray code bit patterns illustrated above to reduce the number of required encoded elements needed to produce a Gray code output having a given resolution. Figures 3, 4 and 5 are schematic representations of toothed wheels which serve as encoded elements for producing the Gray code bit sequences of Figures 1 and 2.

[0019] Wheel 26 in Figure 3 is designed to be mounted on a rotating shaft which would pass through opening 28 and would be used in a position detector that detects the angular position of the shaft. The wheel includes six sectors each having a plurality of teeth 30, 32 and 34 which are sized and spaced to serve as indicia representa­tive of bit sequence 10 in Figure 1. Wheel 26 is divided into six identical sectors and would be useful in the control of a 12 pole dynamoelectric machine since that electrical machine would include six electrical revolutions for each mechanical revolution. Wheel sector 36 is used to produce two identical, but shifted, bit patterns in a four bit Gray code corresponding to the 16 shaft position indications generated as the wheel rotates past sensors 38 and 40 in the direction indicated by arrow 42. With the sensor positions illustrated in Figure 3, sensor 38 produc­es bit pattern 10 of Figure 1 and sensor 40 produces bit pattern 12 of Figure 1. As illustrated, both sensors are initially shown between teeth such that they initially give a logic zero output corresponding to shaft position indica­tion zero in Figure 1. As wheel 26 rotates, sensors 38 and 40 will produce identical bit sequence outputs. The shift in these bit sequences illustrated in Figure 1 is produced by the mechanical spacing between sensors 38 and 40. In this embodiment, that spacing corresponds to four shaft position indication numbers or 1/4 of the angle encompassed by sector 36.

[0020] Figure 4 illustrates a toothed wheel 44 having encoded indicia in the form of peripheral teeth 46 which are spaced such that as wheel 44 rotates in the direction illustrated by arrow 48, sensor 50 will produce bit se­quence 14 and sensor 52 will produce bit sequence 16. To achieved the proper bit sequence shift, sensors 50 and 52 are spaced apart by a distance corresponding to 12 shaft position indication numbers.

[0021] The arrangement of Figure 4 is also used to produce bit sequences 22 and 24 of Figure 2. To produce bit sequences 18 and 20, the combination of wheel 54 and sensors 56 and 58 is used. The wheel 54 includes six sectors each having peripheral teeth 60, 62 and 64. The widths of teeth 60 and 62 correspond to two shaft position indication numbers while the widths of teeth 64 correspond to four shaft position indication numbers. These teeth are spaced along the periphery of wheel 54 such that the distance between teeth represents the logic zero in the shaft position indication numbers illustrated in bit sequences 18 and 20. Wheel 54 rotates in the direction indicated by arrow 66.

[0022] Figures 6, 7 and 8 illustrate alternative encoded elements which may be used in the present invention for the detection of a linear change in position of a movable member. These elements would be attached or otherwise coupled to a linearly movable member. They correspond directly to the toothed wheels of Figures 3, 4 and 5 with corresponding elements being designated by primed numbers. The width of the teeth and the distances between those teeth in Figures 6, 7 and 8 correspond directly to the production of bit sequences 10, 14 and 18 in Figures 1 and 2.

[0023] It should now be apparent to those skilled in the art that Gray codes GC1-GC16 in Tables I and II can be produced by the use of two encoded elements with two sensors positioned adjacent to each of those elements, wherein the sensors are spaced apart by a distance corre­sponding to the shift between bit sequences in correspond­ing bits of the Gray codes.

[0024] Other four bit Gray codes exist wherein only two of the four bits contain identical but shifted bit sequenc­es. Table IV illustrates one such four bit Gray code.

TABLE IV

Four Bit Gray Code Having Two Bit Sequences Of Eight Zeros and Eight Ones, Shifted By Four Numbers
N	DCBA
0	0000
1	0001
2	0011
3	0010
4	0110
5	0111
6	0101
7	0100
8	1100
9	1101
10	1111
11	1110
12	1010
13	1011
14	1001
15	1000

[0025] In the Gray code of Table IV, bits C and D include sequenc­es of eight zeros and eight ones, shifted by four numbers N. It is therefore possible to use a single encoded element with two associated sensors to produce bits C and D and two additional encoded elements, each with its own sensor to produce bits B and A. Figures 9, 10 and 11 illustrate wheels which accomplish this function.

[0026] In Figure 9, wheel 68 mounted on shaft 70 in­cludes an elevated portion 72 corresponding to eight sequential one bits per revolution and a second portion 74 corresponding to eight sequential zero bits. As the wheel 68 rotates in the direction indicated by arrow 76 past sensors 78 and 80, the bit sequences for bits D and C of the Gray code of Table IV are produced. Similarly, in Figure 10 the raised portions 82 and 84 on wheel 86 produce the ones in bit B, while the depressed portions 88 and 90 produce the zeros in bit B as wheel 86 turns past sensor 92 in the direction indicated by arrow 94. Raised sections 96, 98, 100 and 102 of wheel 104 in Figure 11 produce the ones of bit A in Table IV and the recessed sections 106, 108, 110 and 112 of wheel 104 produce the zeros of bit A in Table IV as wheel 104 turns past sensor 114 in the direc­tion indicated by arrow 116.

[0027] In the illustrated preferred embodiments, reluc­tance type sensors which are known in the art would be used in combination with toothed wheels or linear toothed elements constructed of ferromagnetic or other material which produces a change in an electrical characteristic of the sensor. This change is processed in accordance with known techniques to produce the digital output signals used to construct the Gray code.

[0028] Although the present invention has been described in terms of what are at present believed to be its pre­ferred embodiments, it will be apparent to those skilled in the art that various changes may be made to these embodi­ments without departing from the scope of the invention. For example, instead of using a magnetic type encoded element and sensor arrangement, optical sensors could be used in combination with an encoded element having sections of varying reflectivity or transmittance, such as found in known encoder structures. Similarly, magnetic voltage type sensors could be used in combination with magnetic encoded elements. It is therefore intended that such structures are also encompassed by the following claims.

[0029] It will also be apparent to those skilled in the art that the advantages of this invention may also apply to Gray codes having more than four bits. It is intended that the application of this invention to such larger codes is also encompassed by the following claims.

Claims

1. A position detector for providing an indica­tion of the position of a movable member, said detector comprising: a first encoded element (26), having first indicia (30, 32, 34) representative of a first bit sequence of a first pair of bits of a Gray code and capable of being mechanically coupled to a movable member; first and second sensors (38, 40) positioned adjacent to said first encoded element, said first and second sensors producing digital signals in response to said first indicia of said first encoded element; a second encoded element (44), having second indicia (46) representative of a second bit sequence of a second pair of bits of said Gray code and capable of being mechanically coupled to said movable member; and third and fourth sensors (50, 52) positioned adjacent to said second encoded element, said third and fourth sensors producing digital signals in response to said second indicia of said second encoded element characterized in that said first and second sensors are mechanically spaced from each other by an amount corresponding to a shift of said first bit sequence between said first pair of bits; and said third and fourth sensors are mechanically spaced from each other by an amount corresponding to a shift of said second bit sequence between said second pair of bits.

2. A position detector as recited in claim 1, further characterized in that said first bit sequence includes a repetitive succession of two zero bits, three one bits, three zero bits, two one bits, three zero bits, and three one bits; and said second bit sequence includes a repetitive succession of eight zero bits and eight one bits.

3. A position detector as recited in claim 1, further characterized in that said first bit sequence includes a repetitive succession of two zero bits, two one bits, two zero bits, three one bits, four zero bits, and three one bits; and said second bit sequence includes a repetitive succession of eight zero bits and eight one bits.

4. A position detector as recited in claim 1, further characterized in that said first bit sequence includes a repetitive succession of two one bits, two zero bits, two one bits, three zero bits, four one bits, and three zero bits; and said second bit sequence includes a repetitive succession of eight zero bits and eight one bits.

5. A position detector as recited in claim 1, further characterized in that said first indicia includes a first plurality of teeth, said first plurality of teeth being sized and spaced to represent said first bit se­quence; and said second indicia includes a second plurali­ty of teeth, said second plurality of teeth being sized and spaced to represent said second bit sequence.

6. A position detector for providing an indica­tion of the position of a movable member, said detector comprising: a first encoded element (68), having first indicia (72, 74) representative of a first bit sequence of a first pair of bits of a Gray code and capable of being mechanically coupled to a movable member; first and second sensors (78, 80) positioned adjacent to said first encoded element, said first and second sensors producing digital signals in response to said first indicia of said first encoded element; a second encoded element (86), having second indicia (82, 84, 88, 90) representative of a second bit sequence of a third bit of said Gray code and capable of being mechanically coupled to said movable member; and a third sensor (92) positioned adjacent to said second encoded element, said third sensor producing digital signals in response to said second indicia of said second encoded element, characterized in that said first and second sensors are mechanically spaced from each other by an amount corresponding to a shift of said first bit sequence between said first pair of bits.

7. A position detector as recited in claim 6, further characterized by a third encoded element (104), having third indicia (96, 98, 100, 102, 106, 108, 110, 112) representative of a third bit sequence of a fourth bit of said Gray code and capable of being mechanically coupled to said movable member; and a fourth sensor (114) positioned adjacent to said third encoded element, said fourth sensor producing digital signals in response to said third indicia of said third encoded element.

8. A position detector as recited in claim 7, further characterized in that said first bit sequence includes a repetitive sequence of eight zero bits and eight one bits; said second bit sequence includes a repetitive sequence of four zero bits and four one bits; and said third bit sequence includes a repetitive sequence of two zero bits and two one bits.

9. A position detector as recited in claim 6, further characterized in that said first indicia includes a first plurality of teeth, said first plurality of teeth being sized and spaced to represent said first bit se­quence; and said second indicia includes a second plurali­ty of teeth, said second plurality of teeth being sized and spaced to represent said second bit sequence.

Drawing